1. Field of the Invention
The present invention relates to a semiconductor device including a system for charging electric power storing means with electric power generated by electric power generating means, and more particularly, to an overcharge prevention circuit for preventing overcharge of the electric power storing means.
2. Description of the Related Art
FIG. 7 is a circuit diagram of a semiconductor device including a conventional charging system. The semiconductor device includes a solar battery 11 as electric power generating means, a secondary battery 12 as electric power storing means, a diode 13 as a backflow prevention circuit, a voltage detection circuit 14, and switch means 15. The voltage detection circuit 14 and the switch means 15 form an overcharge prevention circuit.
The backflow prevention diode 13 is connected in a charging path from the solar battery 11 to the secondary battery 12, and the switch means 15 is connected in parallel with the solar battery 11.
The voltage detection circuit 14 which is driven by the secondary battery 12 monitors the voltage of the secondary battery 12 and, when the voltage reaches an overcharge voltage, turns on the switch means to short out the solar battery, thus preventing overcharge. When the voltage detection circuit 14 does not detect overcharge, the secondary battery 12 is charged in accordance with the electric power generated by the solar battery 11. Operation of the backflow prevention diode 13 prevents backflow from the secondary battery 12 to the solar battery 11 (see, for example, Japanese Patent Application Laid-open No. Hei 10-336914).
However, the semiconductor device including the conventional charging system drives the voltage detection circuit 14 by the secondary battery 12, and thus, there is a problem that the power consumption of the secondary battery 12 is large. Although not illustrated in the figure, a load circuit which is driven by the secondary battery 12 is designed to operate with extremely low power consumption for the purpose of extending the life of the battery, and thus, it is a critical defect that extra electric power is consumed by the voltage detection circuit 14.
Further, the backflow prevention diode 13 is provided in the charging path, and thus, in charging, voltage loss is produced by the amount of a forward voltage of the backflow prevention diode to deteriorate the charging efficiency.